This invention relates to high-pressure diecasting methods and apparatus, and more particularly, to hot sprue systems for use with hot-chamber, high-pressure diecasting.
There is a very large installed base of hot-chamber, high-pressure, diecasting machines dedicated to the production of small die-cast products of zinc, lead, tin magnesium, aluminum and their alloys. FIG. 1 shows a typical machine 10 of this type. A pool of molten metal 12 is held in a heated pot 14 from which xe2x80x98shotsxe2x80x99 of melt are forced, by a plunger 16 working in a submerged cylinder 18, through a gooseneck 20 and an externally flame-heated connecting nozzle 22, into a cavity 24 formed between a fixed die 26 and a moving die 28. Fixed die 26 is mounted on a fixed platen 30 and moving die 28 is mounted on a moving platen 32 that is pressed toward the fixed platen by the piston 34 of a hydraulic or pneumatic ram, the clamping force being taken by tie-rods 36. When dies 26 and 28 are closed, plunger 16 is driven downwards into cylinder 18 by the piston 38 of a pneumatic ram 40 to force a shot of melt into cavity 24 and to hold the pressure until it has frozen. After which, plunger 16 is raised to suck the remaining liquid melt from nozzle 22 and gooseneck 20 back into melt pool 12. To assist the flowback of melt at the end of a shot, nozzle 22 normally slopes upward to fixed die 26. Indeed, for the same purpose, the whole press portion of the machine (comprising the dies and platens) can be tilted slightly downward towards nozzle 22.
At the commencement of a shot, the melt is conveyed from heated nozzle 22 through a sprue bush 42 fitted in the back of fixed die 26 and through a sprue channel 44, formed in the fixed die 26, to the interface or parting-line 46 of dies 26 and 28. It is then conveyed along the interface 46 by one or more runner channels 48, through a gate orifice 49, into cavity 24. As the injection pressure in such machines is commonly between 10 and 30 mPa, nozzle 22 must be pressed hard against sprue bush 42 and gooseneck 20 to avoid leakage. The use of sprue bush 42 assists in forming the seal at the die end of nozzle 22 and it has the advantage that it can be easily replaced should a freeze-plug form therein after a shot.
Sprue channel 44 is strongly tapered so that it widens toward die interface 46 from sprue bush 42 in the direction of melt flow. It is of such a volume that the freeze-point will occur in sprue channel 44 inwards or down-stream of bush 42 at the end of a shot. On the other hand, the runner channel(s) generally narrow(s) in the direction of melt flow (i.e., towards gate 49) so that the melt is accelerated and enters the cavity at high velocity. This common arrangement of sprue and runner channels allows the cast sprue and runner(s) to be easily removed from the dies, together with the attached products, as one piece after the dies have opened. Each runner channel 48 is normally connected to its respective cavity 24 via narrow slot-like gate 49 so as to form a thin and easily broken connection between the product casting and it""s attached runner and sprue castings.
It will be appreciated from the above that, in this specification, the sprue and runner channels form a melt path within the dies that conveys the melt to the cavity gate(s). The sprue channel conveys the melt from the exterior (normally the back) of the fixed die to the front facexe2x80x94or parting-linexe2x80x94of the fixed-die, while each runner conveys the melt from the sprue channel to the respective cavity gate along the interface between the fixed and moving dies. The sprue and runner castings are the die-cast metal that solidifies in the sprue and runner channels (respectively) at the end of a shot.
Though hot-chamber diecasting is very common, relatively trouble-free and can produce high quality product at high production rates, a major disadvantage of the technique is the large amount of metal contained in the sprue and runner castings compared with the metal in the product. After detachment from the products, the sprue and runner castings are generally remelted and reused, but this represents high-energy losses and causes melt contamination. Another significant disadvantage of conventional hot-chamber diecasting is the abrupt discontinuity in both section and direction in the melt path between the wide and widening sprue channel and the narrow and narrowing runner channel(s); a discontinuity which leads to turbulent and inefficient melt flow.
It will be appreciated that hot-chamber diecasting is a similar process to the injection moulding of plastics materials. While both can pump shots of melt into cavities via sprue and runner systems, losses associated with the sprue and runner castings are much less with injection moulding. In injection moulding, it is common to employ electrically heated sprue-channels (often called xe2x80x98nozzlesxe2x80x99 in the injection moulding context), or electrically heated cores (called xe2x80x98hot-tipsxe2x80x99) within the sprue-channels, to eliminate the generation of sprues. Indeed, if such a device is used to inject molten plastic directly into a cavity, both runners and sprues can be eliminated. It is even possible to use a mechanical valve in the sprue nozzle or hot-tip to close the channel at the entrance to the cavity so that the molten plastic feed-line can be kept pressurized between shots, allowing very high production rates.
While it has been suggested from time to time (see for example U.S. Pat. Nos. 4,304,544 and 4,795,126 to Crandell) that heated nozzles and hot-tips designed for injection moulding can be used for direct-injection diecasting, this has proved impractical. The much higher melting point, thermal conductivity and electrical conductivity of metals relative to plastics have made direct-injection diecasting problematic.
The most notable attempt at direct-injection in hot-chamber diecasting known to the applicant is that undertaken by the Battelle Columbus Laboratories for the International Lead Zinc Research Organization [ILZRO] during most of the 1980s. A large number of progress reports were prepared and published on this work by ILZRO or Battelle. An early such report was published in a paper (No. G-T83-066) entitled xe2x80x9cHeated Manifold, Direct-Injection System for Zinc Diecastingxe2x80x9d by Groeneveld and Kaiser of Battelle and Herrschaft of ILZRO at the International Diecasting Congress and Exposition, Minneapolis, Oct. 31 to Nov. 3, 1983. A further progress report, entitled xe2x80x9cCommercial Application of the Heated-Manifold Direct-Injection System of Zinc Diecastingxe2x80x9d was published in a paper at the Exposition of Jun. 3-6 1985 in Milwaukee, Wiss., with Groeneveld of Battelle as primary author. A further progress report (No. 30) on the ILZRO direct-injection project (authored by Groeneveld) was published by in March 1988, noting that xe2x80x9cseveral million castings have been made by direct injectionxe2x80x9d. Production rates and product quality were reported to be at least equal to conventional diecasting using runners and sprues.
Despite the obvious and great benefits offered by direct-injection diecasting, the technique disclosed in the above publications (particularly, the Battelle work) has not been widely applied by the diecasting industry. The principal reason for this appears to be that die and xe2x80x98nozzlexe2x80x99 design methods for direct injection have not been developed to anywhere near the same facility and reliability as die, runner and sprue design techniques for conventional hot-chamber diecasting. Consequently, a great deal of highly-expert and highly-expensive experimentation must be undertaken before any given product, cavity, die, machine and xe2x80x98nozzlexe2x80x99 combination can be made to work satisfactorily. Furthermore, direct-injection in multi-cavity dies involves major changes to existing diecasting machines with respect to metal flow and control, making machine set-up and tool-changing lengthy processes. In short, implementation of direct-injection diecasting appears to be beyond the technical ability as well as the financial capacity of the great majority of hot-chamber die-casters.
The present invention is based upon the realization that a significant part of the benefit offered by direct-injection diecasting can be achieved with very little change to current die design and no change to hot-chamber machine layout, if a heated sprue channel is employed with a substantially conventional runner channel and if a curved transition channel connects the sprue channel to the runner channel. The temperature of the sprue channel can be controlled to ensure that the melt can run back from the sprue channel after each shot, while the temperature of the transition channel can be arranged so that the freeze-point occurs therein. If the die parting-line includes the transition channel, the casting formed therein (integral with the runner casting) can be ejected with the runner casting in the normal manner. The use of separate mating die inserts to define the transition channel enables the temperature of that channel to be controlled independently of the sprue insert and die temperature. One of the die inserts is preferably a heated sprue body insert in the fixed die, while the other is preferably an opposing, mating and cooled sprue Up insert in the moving die.
Such a xe2x80x98spruelessxe2x80x99 diecasting technique can avoid the production, recovery and remelting of sprues (and so achieve most of the savings of direct-injection) with minimal change to die design, no change to machine layout and without any need for troublesome valved injection nozzles. Furthermore, the use of a hot-sprue die-insert allows the melt flow-path within the die to be optimized for streamlined flow without the need for any abrupt change of section. Indeed, gradual and uniform tapering of the melt path is readily achieved from the sprue inlet to the gate so that the melt is constantly and smoothly accelerated. The need for a reverse taper on the sprue channel to permit withdrawal of a sprue casting is, of course, eliminated.
It will be normal for the sprue channel to extend roughly horizontally in the fixed die so as to be orthogonal to the die parting-line and the back of the fixed die (as is conventional) and for the transition channel to be a smooth curve that subtends an angle of about 90xc2x0 from the sprue channel to the runner channel. However, the sprue and/or the die parting-line may be oriented so that the angle subtended by the curve of the transition channel is other than 90xc2x0, though this will be unusual. As already noted, the transition channel can be made to decrease (taper) in cross-section in a smooth and uniform manner so that the melt is accelerated as it flows around the curve to enter the runner channel.
It is preferable that the insert in the moving die that defines portion of the transition channel has its own cooling means (such as provision for coolant circulation) so that its temperature can be adjusted independently of the rest of the moving die so as to ensure that freeze-off occurs in the transition channel. For this purpose, it is desirable for the insert to include temperature sensor means so that automatic regulation can be effected. Similarly, it is desirable for the sprue-insert in the fixed die to include temperature sensor means so that it can be kept at a sufficiently high temperature to ensure flowback of the melt after a shot.
It will be appreciated that mating die inserts like those required to define the transition channel are familiar to average diecasting toolmakers and that the setting of appropriate temperatures for the sprue and transition channels to achieve appropriate flowback and freeze-off (respectively) are well within the competency of average machine operators. Apart from the need for the die inserts, tool design and machine operation normally will be unaffected by the method of the present invention. Normal cavity inserts and ejection mechanisms can be used. Where there is more than one cavity in a die-set branching or radiating transition channels can be formed in the inserts to connect the sprue channel to the various runner channels.
It will be seen that the present invention relates to diecasting methods, diecasting apparatus and to die-inserts for use in diecasting.
Having portrayed the nature of the present invention, two examples will now be described with reference to the accompanying drawings. However, those skilled in the art will appreciate that many variations and modifications can be made to the chosen examples without departing from the scope of the invention as defined by the following claims.